Femtocell base stations in a Long Term Evolution (LTE) communication network (otherwise known as Home evolved Node Bs—HeNBs- or Enterprise evolved Node Bs-EeNBs) are small, low-power, indoor cellular base stations for residential or business use. They provide better network coverage and capacity than that available in such environments from the overlying macrocellular LTE network. In addition, femtocell base stations use a broadband connection to receive data from and send data back to the operator's network (known as “backhaul”).
Femtocell base stations can operate in one of three access modes known as “closed”, “open” or “hybrid”. For femtocell base stations operating in a “closed” access mode, only a limited set of mobile devices (otherwise known as User Equipments—UEs) belonging to the network operator and associated or registered with the femtocell base station are allowed to access the femtocell base station. This set of UEs is called the Closed Subscriber Group (CSG), and would typically consist of family members where the femtocell base station is used in a home environment, or employees where the femtocell base station is used in an enterprise or work environment.
For femtocell base stations operating in an “open” access mode, all UEs belonging to the network operator are allowed to access the femtocell base station. This might be desirable in order to allow other UEs that would otherwise have a poor signal quality, or that would otherwise cause or suffer interference to/from the femtocell base station, to access (i.e. hand-in to) the femtocell base station.
For femtocell base stations operating in a “hybrid” access mode as defined in 3GPP release 9 (for example see 3GPP TS 36.300), all UEs belonging to the network operator are allowed to access (i.e. hand-in to) the femtocell base station as in the “open” mode. However once the UEs access the femtocell base station, priority is given to UEs that are in the CSG set in order to avoid significant degradations in quality of service that might result from the UEs that are not in the CSG set consuming too many resources of the femtocell base station (these resources might include femtocell base station power, bandwidth (in LTE networks) or code resource (in WCDMA networks), and backhaul bandwidth). These non-CSG UEs are referred to as “visiting UEs” herein as they are temporarily making use of the resources of the femtocell base station. The UEs that are making use of the macrocell base station rather than the femtocell base station are referred to as “macro UEs” herein, and the UEs that are part of the Closed Subscriber Group of the femtocell and that are making use of the femtocell base station are referred to as “home UEs”.
Conventionally, the decision for performing a handover of a macro UE from a macrocell base station to a femtocell base station (and vice versa, i.e. performing a handover of a home or visiting UE from the femtocell base station to the macrocell base station) is based on the occurrence of one or more of a number of “measurement events”, that are indications that a specific event has occurred. These measurement events are used to trigger the sending of measurement reports from a UE to the current serving cell (i.e. the macrocell for a macro UE and femtocell for a home or visiting UE). The measurement events specified in the 3GPP specification include:                Event A1—The signal quality of the serving cell becomes better than a threshold.        Event A2—The signal quality of the serving cell becomes worse than a threshold.        Event A3—The signal quality of a neighbouring cell becomes an offset amount better than the signal quality in the serving cell.        Event A4—The signal quality of a neighbouring cell becomes better than a threshold.        Event A5—The signal quality of the serving cell becomes worse than a first threshold and the signal quality in a neighbouring cell becomes better than a second threshold.        
The measurement reports sent from the UE to the current serving cell may include Received Signal Reference Power (RSRP) and Received Signal Reference Quality (RSRQ) measured on both source and target cells. RSRP is a measurement of received power made on the reference signals transmitted in a cell whereas RSRQ is a measure of quality. Both are measurements of the downlink. The decision to access a target cell is typically made based on RSRP such that a decision to handover will only be made if the
RSRP on the target cell is better by some threshold. If this threshold equals the threshold used in the measurement event then in effect the measurement event triggers the handover.
It has been noted that the use of femtocell base stations that may be useable by any UE of the network, i.e. use of femtocell base stations operating in the open access node or the hybrid access mode, has the potential to improve the overall performance of the network (over both the macrocell and femtocell layers) as follows:                coverage problems in the macrocell for macro UEs can be alleviated by allowing the macro UEs to access a nearby femtocell;        downlink interference from a nearby femtocell to a macro UE downlink (i.e. interference to transmissions from the macrocell base station to the macro UE from transmissions between the femtocell base station and home or visiting UEs) can be alleviated by allowing the ‘victim’ macro UE to access the femtocell; and        uplink interference from a nearby macro UE to a femtocell (i.e. interference to transmissions between the femtocell base station and home or visiting UEs from transmissions from macro UEs that are within the coverage area of the femtocell base station) can be alleviated by allowing the macro UE to make use of the ‘victim’ femtocell base station.        
However, there are problems with using the conventional handover measurement procedure to decide whether to allow a macro UE to make use of (i.e. hand-in to) a femtocell base station, especially one operating in a hybrid access mode, (and vice versa, i.e. whether a visiting UE should still be allowed to make use of the femtocell base station).